This invention relates to materials useful with release coatings, that is, coatings which release materials that would ordinarily be adherent. In particular, this invention relates to adhesion-enhancing additives for release coatings.
When labels, stamps, decorative laminates, transfer tapes, or the like are coated with an adhesive, it is desirable that the substrate, on which the labels (for example) are stored be easily peeled away when the label is ready for use. However, it is also important that the adhesive quality of the label not be derogated by removal from the substrate. This can be accomplished by coating the substrate with a release coating, commonly a silicone release coating. Silicone release coatings on paper, polyethylene, Mylar and other substrates also provide non-stick surfaces for food handling and industrial packaging applications.
Epoxy functionalized polyorganosiloxanes have proved especially useful as release coatings. While these polymers show good adhesion to substrates such paper, they show poorer adhesion to plastic films such as polyethylene, polypropylene, polystyrene and polyester. Release coatings must have good adhesion to the substrate, or the coating may be removed during the manufacturing process or along with the adhesive when the label is removed from the substrate. One method to improve adhesion to plastic films is to employ an adhesion promoter, also known as an anchorage additive. However, few adhesion promoters are known.
U.S. Pat. No. 5,567,764 discloses the use of trialkoxysilane or trialkoxysiloxane containing alkenyl functional polyorganosiloxanes as adhesion promoters for addition cure release coatings. In addition cure release coatings, crosslinking is effected in the presence of a metal catalyst, such as platinum, palladium, rhodium, ruthenium, or iridium. This is in contrast to ultraviolet (UV) cure release coatings, which are cationically crosslinked in the presence of a UV light and a photoinitiator. When 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, and vinyltriacetoxysilane are used in UV cure release coatings, the achieved anchorage is not consistent and the release coatings rub off. Accordingly, there remains a continuing need in the art for adhesion promoters which will provide consistent anchorage of UV cure release coatings to substrates, particularly to plastic films.